1. Field of the Invention
The present invention relates to a method of building a tyre, in particular for two-wheeled vehicles, comprising the steps of: forming a carcass structure on a toroidal support which carcass structure comprises at least one carcass ply having its ends in engagement with respective circumferentially-inextensible annular anchoring structures; applying a belt structure at a radially external position with respect to the carcass ply; applying a pair of sidewalls at an axially external position relative to side surfaces of the carcass ply each extending radially away from one of the annular anchoring structures towards said belt structure; applying a tread band to the belt structure at a radially external position.
The present invention also relates to a tyre, in particular for two-wheeled vehicles comprising: a carcass structure provided with at least one carcass ply having its ends in engagement with respective circumferentially-inextensible annular anchoring structures; a belt structure applied at a radially external position with respect to the carcass ply; a pair of sidewalls applied at an axially external position to side surfaces of the carcass ply each extending radially away from one of the annular anchoring structures towards said belt structure; a tread band applied to the belt structure at a radially external position.
2. Description of the Related Art
Building of tyres for two-wheeled vehicles in general involves formation of a carcass structure essentially made up of one or more carcass plies substantially shaped in a toroidal configuration and having their axially opposite side edges in engagement with respective annular reinforcing structures incorporating circumferentially inextensible annular elements, usually referred to as “rings”.
Applied to the carcass structure, at a radially external position, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially made up of textile or metallic cords which are suitably oriented with respect to each other and to the cords belonging to the adjacent carcass plies.
In addition, a tread band is applied to the belt structure at a radially external position, said tread band being usually made up of a strip of elastomer material of appropriate thickness.
To the aims of the present specification it should be pointed out that by the term “elastomer material” it is intended a rubber blend in its entirety, that is the combination of at least one base polymer suitably amalgamated with reinforcing fillers and process additives of various types.
Finally, a pair of sidewalls is applied to the opposite sides of the tyre being formed, each of said sidewalls covering a side portion of the carcass structure included between a so-called shoulder region, located close to the corresponding edge of the tread band, and a so-called bead located at the corresponding annular reinforcing structure.
While tyres for cars or trucks are characterised by a substantially flattened cross-section outline at the tread band, tyres for motorcycles are distinguishable due to their marked transverse curvature or bending, usually defined by the particular value of the ratio between the distance of the tread centre from the line passing through the opposite side extremities of the tread itself, measured at the equatorial plane of the tyre, and the distance measured along the tyre chord between said extremities. In tyres for two-wheeled vehicles the value of the bending ratio generally is at least as high as about 0.15 and it is usually in the order of 0.3 in the case of rear tyres, and even higher, until 0.45 in the case of front tyres, against a value usually smaller than 0.05 in tyres for motor-vehicles.
Leaving the destination of use out of consideration, i.e. irrespective of its being used for motor-vehicles or motor-cycles, tyres are generally classified in at least two categories which are different from each other due to the orientation of the cords being part of the carcass plies. In particular, there are the so-called “radial tyres” in which each of the cords arranged in the carcass ply or plies lies in a plane substantially radial to the rotation axis of the tyre, i.e. it has an orientation substantially orthogonal to the circumferential extension direction. In addition, there are tyres of the so-called “traditional type” or “crossed-ply tires” the carcass structure of which generally comprises at least a first carcass ply having cords oriented obliquely to the circumferential extension direction of the tyre, and a second carcass ply the cords of which have an oblique orientation which is symmetrically crossed with respect to the cords of the first ply.
As compared with tyres of the conventional type, radial tyres offer advantages in terms of lightness, ride comfort and structural strength at high speeds. Due to this circumstance, tyres of the conventional type practically fell in disuse in favour of tyres of the radial type, at least with reference to their use on cars and in the countries where a modern road network is present.
With reference to motorcycles as well, use of tyres of the radial type has recently imposed itself, in particular on motorcycles of recent conception involving use of low-section tyres, i.e. tyres in which the ratio of the section height, measured between the bead base and the tread band centre, to the maximum tyre width is, by way of example, less than 0.7.
There are however particular market sectors in which use of tyres of the radial type is presently precluded. This in particular occurs with reference to motorcycles mounting tyres with a rather high section ratio, by way of example greater than 0.7, as those of the so-called “custom” type in which the frame, suspensions and rims are inspired to technical and stylistic solutions going back to fifties and sixties. These motorcycles have recently awoken the interest of an important part of users, and today represent a non negligible portion of the circulating car pool. In terms of tyres, these vehicles both for aesthetic and functional reasons, require use of tyres having a high section ratio, which greatly conditions both the vehicle behaviour on the road and the tyre structure. In fact, these tyres, due to the important height of the sidewall, need carcass structures provided with rather stiff sidewalls to ensure the necessary vehicle steadiness in all use conditions, when running both on a straight stretch and on a bend. In all the above cases, resorting to tyres with a carcass structure of the conventional type, i.e. with several plies having mutually crossed cords that are inclined with respect to the circumferential direction, appears therefore necessary.
The Applicant has now perceived that the above tyres for motorcycles with a high section ratio, traditionally obtained by means of production processes involving assembling of previously made and stored semi-finished products, do not lend themselves to be constructed following production methodologies of recent conception, aiming at eliminating or at least limiting the necessity to produce and store semi-finished products.
An example of these production methodologies is described in document EP-A-0928680, in the name of the same Applicant, where each tyre component is directly made on a rigid toroidal support conforming in shape to the inner conformation of the tyre itself, using a semi-finished product continuously fed from an extruder or other appropriate devices. In more detail, one or more carcass plies are each obtained by laying down strip-like lengths in sequence and in mutual side by side relationship on the toroidal support, said strip-like lengths being obtained by cutting of a continuous semi-finished product directly coming from an extruder and comprising longitudinal cords incorporated in an elastomer layer. Other tyre components, such as the anchoring inserts incorporated in the annular reinforcing structures at the beads, are obtained by winding of a continuous thread-like element of metal material into radially superposed coils. Other components made of elastomer material such as the tread band, sidewalls and others, are obtained by winding up on the carcass structure, a continuous elongated element of elastomer material directly extruded from an extruder so as to form coils disposed in side by side and/or superposed relationship.
By adopting the above production methods important advantages could be achieved both in terms of productivity and in terms of production flexibility of the plants.
The Applicant has however perceived that, with use of such production methods, accomplishment of the traditional carcass structure with several plies having cords of crossed extension involves an important increase in the production times and costs, which neutralizes an important part of the advantages typically correlated with these production methodologies.